I just realized that I missed the Opportunity Maine Tax Credit for 2015. If I file and get a refund do I have to refile my fed return for 2015 or claim it for 2019?

If I receive the refund during 2019 but it was for 2015 tax year, can I just include the refund amount in my 2019 taxes as that is when it was paid?  I really don't want to have to redo my federal returns if I can help it.